This invention relates to vehicle brake control systems and more particularly to control systems for brakes of the kind which are actuated by the application of fluid pressures thereto.
A common form of vehicle braking system employs a master cylinder which may be operated to transmit a fluid pressure to individual brakes at the vehicle wheels for the purpose of applying the brakes. While the master cylinder is operated through mechanical linkages in some systems, heavy vehicles such as trucks or earthmoving apparatus often have a pneumatic circuit for actuating the master cylinder in response to manipulation of an operator's brake control. Typically, a service tank of compressed air may be communicated with the master cylinder through a relay valve in response to a fluid pressure signal originating at the operator's service brake pedal or the like.
Failure of the braking system in a vehicle can be extremely hazardous and thus it is customary to supplement the service braking system with an emergency brake system. In some cases, the emergency brake may be completely independent of the service braking system. In other vehicles a separate emergency compressed air tank is provided which, upon actuation of an emergency brake control, supplies pressure to the same master cylinder that provides service braking. Heavy vehicles may also be equipped with a retarder braking system which partially actuates the master cylinder in response to setting of an operator's retarder control, the degree of braking provided by the retarder system being less than that of the service or emergency system so that the usual effect is to slow motion of the vehicle for travelling down a slope or the like rather than fully stopping the vehicle.
The redundancy provided by the existence of both a service braking system and an emergency braking system and in some cases a retarder braking system substantially reduces the hazards inherent in potential brake failure. However, if the emergency braking system is of the form which is completely independent from the service braking system, considerable complexity and cost are added to the overall system in order to provide a function which in practice may rarely be utilized. If the emergency system is of the form which acts through the same master cylinder that is utilized in the service braking system, then there is no protection against malfunction of the master cylinder itself.
Considering now another aspect of such braking systems, it is customary to provide some means to warn the operator of the vehicle if the pressure within a service or emergency compressed air tank approaches a minimum safe operating level. Prior U.S. Pat. No. 3,601,451 discloses a braking system which provides still more positive protection in that the emergency braking system is automatically activated if the pressure within the emergency tank has dropped to a predetermined minimum safe value because of prolonged continued use of the emergency system or some other cause. Once the emergency system is actuated in this manner it cannot be released until the pressure within the emergency tank has been restored to a normal operating level. Thus, in the system of prior U.S. Pat. No. 3,601,451, sudden braking can occur unexpectedly insofar as the operator is concerned. While this is fundamentally a valuable safety measure, it can also complicate the operator's task. If conditions permit, the operator would generally prefer to bring the vehicle to a more controlled emergency stop by modulated controllable braking rather than by suddenly having the brakes locked on and out of manual control.
Considering still another aspect of braking systems of this general type, response of the brakes to application and release of the operator's braking controls should be very rapid to avoid unwanted lockup of the wheels with consequent tire wear and a risk of skidding. Leakage of air into the hydraulic lines between the master cylinder and brakes is one example of a factor which can slow response, and delayed restoring of a master cylinder after operation is another. The need to move large quantities of hydraulic fluid and air also slows response. Prior braking systems have in general been characterized by undesirably slow response at least under some conditions.